1. Field of the Invention
The present invention relates to a mobile communication system and, in particular, to a signal transmission method and apparatus of a User Equipment (UE) for a mobile communication system that is capable of improving data transmission reliability and efficiency by transmitting data based on the precise discrimination of the uplink transmission resource assignment messages indicative of initial transmission and retransmission.
2. Description of the Related Art
Universal Mobile Telecommunications System (UMTS) is one of the third generation (3G) mobile telecommunication technologies, which evolved from Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS) systems and uses Wideband Code Division Multiple Access (WCDMA).
The 3rd Generation Partnership Project (3GPP), which is responsible for the standardization of UMTS, is working to significantly extend the performance of UMTS in the Long Term Evolution (LTE) system. LTE is a 3GPP standard that provides for a downlink speed of up to 100 Mbps and is expected to be commercially launched in 2010.
In order to fulfill the requirements for the LTE systems, studies have been done in various aspects including minimization of the number of involved nodes in the connections and placing radio protocol as close to the radio channels as possible.
FIG. 1 is a diagram illustrating an LTE mobile communication system.
As shown in FIG. 1, the LTE mobile communication system utilizes the Evolved Radio Access Network (E-RAN) 110 and 112 having only two infrastructure nodes: the Evolved Node B (ENB or Node B) 120, 122, 124, 126, and 128 and the Access Gateway (AG) 130 and 132. A User Equipment (UE) 101 accesses the Internet Protocol (IP) network via the E-RAN 110 and 112.
The ENB's 120, 122, 124, 126, and 128 correspond to the conventional Node B which provides the UE 101 with radio access service. The ENB's 120, 122, 124, 126, and 128 are responsible for more complex functions than that of the conventional Node B. In the next generation wireless communication system, all user traffic including real time services such as Voice over IP (VoIP) are served through a shared channel. For this reason, there is a need for a device to collect status information of the UE's and scheduling based on the status information. Each of the ENBs 120, 122, 124, 126, and 128 is responsible for scheduling the UE's. In order to achieve the speed of 100 Mbps or faster, the wireless communication system exploits the radio access technology of Orthogonal Frequency Division Multiplexing (OFDM) on a 20 MHz bandwidth. Also, an Adaptive Modulation and Coding (AMC) scheme is supported for determining a modulation scheme and a channel coding rate according to the channel status of the UE 101.
FIG. 2 is a diagram illustrating a user plane protocol stack architecture of an LTE mobile communication system.
As shown in FIG. 2, the UE has a protocol stack including a Packet Data Convergence Protocol (PDCP) layer 205, a Radio Link Control (RLC) layer 210, a Media Access Control (MAC) layer 215, and a Physical (PHY) layer 220. Also, the ENB has a protocol stack including a PDCP layer 240, a RLC layer 235, a MAC layer 230, and a PHY layer 225.
The PDCP layers 205 and 240 are responsible for IP header compression/decompression. The RLC layers 210 and 235 pack the PDCP Packet Data Units (PDUs) into a size appropriate for transmission and performs an Automatic Repeat reQuest (ARQ) function. The data unit delivered from an upper layer entity is a PDU. The MAC layers 215 and 230 serve multiple RLC layer entities. The MAC layers 215 and 230 can multiplex the RLC PDUs produced by the RLC layer entities into a single MAC PDU and de-multiplex a MAC PDU into the RLC PDUs. The physical layers 220 and 225 perform encoding and modulation on the upper layer data to transmit it through a radio channel and perform demodulation and decoding on the OFDM symbol received through radio channel to deliver to upper layers.
In the LTE mobile communication system, a Hybrid Automatic Repeat reQuest (HARQ) is utilized for reliable transmission of uplink MAC PDUs. In the mobile communication system using HARQ, when a MAC PDU is not received, the receiver transmits a HARQ Non-Acknowledgement (NACK) to the transmitter such that the transmitter received the HARQ NACK retransmits the MAC PDU. The receiver performs soft combining to combine the initial transmission and the one or more retransmissions.
In the LTE mobile communication system, uplink resources are allocated by means of an uplink transmission resource assignment message.
FIG. 3 is a diagram illustrating a message format of an exemplary uplink transmission resource assignment message.
Referring to FIG. 3, the uplink transmission resource assignment message includes a Resource Block (RB) assignment field 305 carrying information on the amount and position of the resource. In the LTE mobile communication system, the resource is assigned in a unit of a resource block defined with a predetermined frequency bandwidth of a length of 1 msec, and the assigned resources are indicated by the RB assignment field 305.
The uplink transmission resource assignment message also includes a Modulation and Coding Scheme (MCS) field 310 which indicates the adaptive modulation and coding formats to be adopted to the transmission data. This field is 5 bits and can carries one of 29 code points indicating combinations of Quadrature Phase Shift Keying (QPSK) and 0.11 channel coding rate to 64 Quadrature Amplitude Modulation (64QAM) and 0.95 channel coding rate. The rest three code points are used to indicate the Redundancy Version (RV). The MCS field 310 is described in more detail with reference to Table 1.
TABLE 1MCS IndexModulation OrderTBS IndexRedundancy VersionIMCSQ′mITBSrvidx020012102220323042405250626072708280929010210011410012411013412014413016415017416018417019418020419021619022620023621024622025623026624027625028626029reserved1302313
Table 1 is specified in the 3GPP 36.213 V8.6.0 Technical Specification for showing the relationship between Transport Block Size (TBS) index and RV according to the MCS index contained in the MCS field 310. Referring to Table 1, for MCS levels from 0 to 28 indicates RV=0 and respective TBS indices, and the MCS levels from 29 to 31 indicates RV=1, 2, and 3, respectively with no TBS index.
The uplink transmission resource assignment message also includes a New Data Indicator (NDI) 315. The NDI 315 is 1 bit information to indicate whether the transmission resource assignment message is of either an initial transmission or retransmission.
The uplink HARQ operation with the NDI of the uplink transmission resource assignment message is described in FIG. 4.
FIG. 4 is a diagram illustrating a successful HARQ operation in the conventional mobile communication system using the NDI of the uplink transmission resource assignment message.
Referring to FIG. 4, a UE receives an uplink transmission resource assignment message 405 at a point in time and transmits a MAC PDU using the uplink resource 415 and MCS level indicated by the uplink transmission resource assignment message 405 after time T 410 elapses. In order to simplify the explanation, the MAC PDU transmitted through the resource assigned with the uplink transmission resource assignment message 405 will be referred to as MAC PDU A. Until the MAC PDU A is completely transmitted, i.e. before a HARQ ACKnowledgement (ACK) on the MAC PDU A is received or the maximum number of retransmission times is reached, the UE repeats retransmissions 420 and 425 at interval of HARQ Round Trip Time (RTT) 417. The MAC PDU A transmission is completed at a specific point in time, and the UE receives a new uplink transmission resource assignment message 430 from the ENB. The NDI of the new uplink transmission resource assignment message is set to a value different from that of the previous uplink transmission resource assignment message 405. If the NDI value change is detected, then the UE regards the uplink transmission resource allocation message as a new MAC PDU request message and starts transmitting a new MAC PDU, e.g. MAC PDU B, through the uplink resource 435. As aforementioned, the UE repeats transmission 440 at interval of the HARQ RTT.
In order to manage the transmission resources efficiently, the ENB can assign the different resources for retransmission of the current MAC PDU. This technique is called adaptive retransmission. By transmitting the uplink transmission resource allocation message having the same NDI, the ENB can request the UE for adaptive retransmissions. For instance, if the ENB wants to receive the MAC PDU B through a new RB after transmitting the uplink transmission resource assignment message 430, then the ENB transmits an uplink transmission resource assignment message 433 of which NDI (=1) is identical to that of the previous uplink transmission resource assignment message 430 along with the new resource assignment information. If an uplink transmission resource allocation message of which NDI is identical to that of the previous uplink transmission resource allocation message is received, then the UE determines that the adaptive retransmission is requested so as to transmit the MAC PDU B using the newly assigned transmission resource 445 and 450.
To determine whether the uplink transmission resource assignment message is of an initial transmission or retransmission is possible under the assumption that the UE receives all the uplink transmission resource allocation messages are received successfully.
FIG. 5 is a diagram illustrating a HARQ operation failure in the conventional mobile communication system using NDI of the uplink transmission resource assignment message.
Referring to FIG. 5, a UE receives an uplink transmission resource assignment message 505 at a point in time and transmits a MAC PDU using the uplink source 515 and MCS level indicated by the uplink transmission resource assignment message 505 and repeats retransmissions 520 of the MAC PDU at an interval of the HARQ RTT as shown in FIG. 4. After receiving the MAC PDU completely, the ENB transmits a new uplink transmission resource allocation message 525 of which NDI (=1) is different from that (=0) of the previous uplink transmission resource allocation message 505. In the case where the new uplink transmission resource allocation message 525 is lost, the UE may receive another uplink transmission resource allocation message 530 having the NDI (=0) identical to that of the previous uplink resource transmission resource allocation message 505 without being aware of the lost uplink transmission resource allocation message 525 of which NDI is set to 1. Since the NDI (=0) of the uplink transmission resource allocation message 530 is the first message after receiving the previous uplink transmission resource allocation message 505 in view of the UE and the NDIs of the two successively received messages are identical to each other, the UE misidentifies that the uplink transmission resource allocation message 530 is a retransmission request message.